1. Field of the Invention
The present invention relates to a rotary compressor performing multi-stage compression, and more particularly, to a multi-stage rotary compressor capable of optimizing compression efficiency using all compression units.
2. Description of the Background Art
A compressor is a device that increases pressure by compressing the air, a refrigerant gas or other specific gases upon receiving power from a power generator such as an electric motor, and is being used throughout industries. The compressor may be divided into a positive displacement compressor and a turbo compressor according to how to compress. The positive displacement compressor performs compression by a compression method in which pressure is increased through a volume decreased, and the turbo compressor performs compression by converting kinetic energy of gas into pressure energy.
A rotary compressor, which is a kind of positive displacement compressor, is commonly used for an air conditioning apparatus such as an air-conditioner. In response to demands for air-conditioners having various functions, the rotary compressor that can change its capacity is being required in these days.
The rotary compressor has used a refrigerant containing a CFC-based chlorine. However, such a refrigerant is known as a factor causing destruction of the ozone layer, which results in global warming. As a result, its use is legally regulated and extensive researches have been made for an alternative refrigerant with respect to the existing refrigerant. Carbon dioxide is expected as an alternative refrigerant. Moreover, the global warming leads issues of improvement of energy efficiency of a device as well as issues of the alternative of the existing refrigerant.
Naturally, about a compressor considered as the heart of a freezing system, the biggest concern is how alternative refrigerants harmless to global environment can be used in existing compressors without the performance loss.
There is a multi-stage rotary compressor having a plurality of compression units which can change its capacity and use an alternative refrigerant.
FIG. 1 is a sectional view showing one example of the conventional multiple-stage rotary compressor.
As shown, the conventional multiple-stage rotary compressor includes: a casing 1 at which two gas suction pipes 30 and 31 and a gas discharge pipe 40 are installed to communicate with each other; a motor unit 2 installed at an upper side of the casing 1 and including a stator 3 and a rotor 4 for generating a rotary force; and a first compression unit 10 and a second compressor unit 20 installed at upper and lower portions of a lower side of the casing 1 and respectively compressing a refrigerant upon receiving a rotary force generated from the motor unit 2 by a rotary shaft 5.
One accumulator 6 for separating liquefied refrigerant from a suction refrigerant is installed between the gas suction pipes 30 and 31 and between the compression units 10 and 20. The first gas suction pipe 30 supplies a refrigerant to a first cylinder 11 by being connected to a first suction port 17, and the second gas suction pipe 31 supplies a refrigerant to a second cylinder 21 by being connected to a second suction port 27.
The first compression unit 10 includes: a first cylinder 11 formed as a ring shape and installed inside the casing 1; an upper bearing 12 and a middle bearing 13 covering both upper and lower sides of the first cylinder 11, forming a first inner space 19 together, and supporting the rotary shaft 5 in radial and axial directions; a first rolling piston 14 rotatably coupled to an upper eccentric portion of the rotary shaft 5 and orbiting in a first internal space 19 of the first cylinder 11 to thereby compress a refrigerant; a first vane (not shown) coupled to the first cylinder to be movable in a radial direction so as to pressingly contact with an outer circumferential surface of the first rolling piston 14, and dividing the first inner space 19 of the first cylinder 11 into a first suction chamber and a first compression chamber; and a first discharge valve 15 coupled to a front end of a first discharge port 16 provided at the upper bearing 12 to open or close the first discharge port 16, for controlling the discharge of a refrigerant gas.
The second compression unit 20 includes: a second cylinder 21 formed as a ring shape and installed under the first cylinder 11 inside the casing 1; a middle bearing 13 and a lower bearing 22 covering both upper and lower sides of the second cylinder 21, forming a second inner space together, and supporting the rotary shaft 5 in a radial direction and an axial direction; a second rolling piston 23 rotatably coupled to a lower eccentric portion of the rotary shaft 5, and orbiting in the second inner space of the second cylinder 21 to compress a refrigerant; a second vane (not shown) coupled to the second cylinder 21 to be movable in a radial direction so as to pressingly contact with an outer circumferential surface of the second rolling piston 23, and dividing the second inner space 29 into a second suction chamber and a second compression chamber; and a second discharge valve 24 coupled to a front end of a second discharge port 26 provided at the lower bearing 22 to open or close the second discharge port 26, for controlling the discharge of a refrigerant gas discharged from the second compression chamber.
The operation of the conventional multiple-stage rotary compressor having such a structure will now be described.
When the rotor 4 rotates as power is applied to the stator 3 of the motor unit 2, the rotary shaft 5 rotates together with the rotor 4, transferring a rotary force of the motor unit 2 to the first compression unit 10 and the second compression unit 20. Thus, a refrigerant gas is sucked and compressed in the inner spaces 19 and 29 of the compression units 10 and 20 by the rolling pistons 14 and 23 and the vane (not shown). At this time, in the first compression unit 10 and the second compression unit 20, suction, compression and discharge strokes are alternately performed with a phase difference of about 180 degrees.
Such an ordinary multi-stage rotary compressor sequentially performs suction, compression and discharge of a refrigerant as the rolling piston contacts with an inner diameter of the cylinder at one point. In order to generate many loads and thereby obtain a high capacity (hereinafter, referred to as power mode), the compression units are respectively driven. At this time, the capacity of the compressor would be the sum total of refrigerant discharged from each compression unit. In order to obtain power saving effect with a low capacity due to a reduced load (hereinafter, referred to as saving mode), refrigerants sucked into some compression units are cut off, or the vane moves back and is fixed by a piece or the like, thereby removing a boundary between the suction chamber and the compression chamber, so that the rolling piston does not compress refrigerant but is idled.
As another method of implementing the saving mode, the capacity of the refrigerant is changed by speed changes using an inverter motor having a control drive as a driving unit.
The structure of the ordinary rotary compressor and a driving method therefor have the following problems.
First, the method in which the vane is moved back and fixed in the saving mode has problems that a special part such as a piece or the like and a space where the part is mounted are undesirably required, and the number of manufacturing processes increases.
Secondly, as the piece repetitively applies an impact on the vane, a surface of the vane may be damaged as time goes on, and reliability issues such as abrasion, foreign substance generations and the like may be caused.
Thirdly, using an inverter motor as a driving unit may bring about an increase in manufacturing cost since the inverter motor is expensive in general. Accordingly, there is a need to implement capacity changes with a relatively cheap constant speed motor.
Fourthly, when an existing constant speed motor is used, ON/OFF operation is frequently repeated for a room temperature control. For this reason, power consumption is great due to a starting current, and abrasion of a compression unit increasingly occurs, which results in degradation of reliability of a compression unit. Also, since a variation between a set temperature and a room temperature is great in ON/OFF of the constant speed motor, it is difficult to control the room temperature for a delight condition of a room.
Fifthly, when the compression unit is idled or suction of the refrigerant is prevented, some compression units are not used at all, which degrades efficiency of the compressor.